Conventional subsonic transport aircraft typically include wing mounted gas turbine engines. The engines are typically mounted below the wing by using conventional pylons and are surrounded by an annular conventional nacelle for providing an aerodynamically smooth envelope. Since weight in an aircraft is an important consideration, it is desirable that the nacelle be as small as possible and as light as possible for reducing weight and aerodynamic drag due to the flow of freestream airflow thereover and therethrough. Accordingly, the length, diameter, and thickness of the nacelle directly affects both the weight of the nacelle and the drag area of the nacelle.
Furthermore, the nacelle is typically designed for having an internal flow passage which provides freestream airflow to the gas turbine engine with varying degrees of aerodynamic efficiency. Operation of the nacelle is affected by adjacent structures such as the aircraft fuselage and wing, and different design considerations exist for both low speed operation of the aircraft, such as for example during takeoff, and high speed operation of the aircraft, such as cruise, for example.
Typical aerodynamic performance parameters for evaluating low speed operation of the nacelle include total pressure recovery, circumferential pressure distortion, angle of attack capability of the nacelle relative to freestream airflow without flow separation, and crosswind effects acting on the nacelle. At cruise operation of the aircraft, the incidence angle of the freestream airflow relative to the nacelle is an important consideration relative to avoiding excessive drag. Other cruise considerations include sensitivity of drag along the external surface of the nacelle to changes in engine airflow and freestream Mach number.
Furthermore, Government regulations typically limit the amount of acceptable noise which may be directed to the ground by the gas turbine engines during low speed, takeoff operation. Conventional nacelle inlets require acoustic treatment within the nacelle for meeting noise regulations and require relatively thick nacelle lower lips for meeting low speed high angle of attack requirements for obtaining acceptable flow separation margin. Both of these requirements add weight to the nacelle and the relatively thick lower lip also increases drag. Conventional inlets are typically drooped wherein the nacelle diffuser has a centerline droop axis which is inclined relative to the engine centerline axis so that the nacelle inlet face is perpendicular to the freestream airflow at cruise for minimizing installed drag over the nacelle. Such a droop nacelle inlet also enjoys improved low speed high angle of attack performance.
An advanced concept nacelle inlet sometimes referred to as a scarf, or scoop, inlet is one wherein its lower lip extends forward of the upper lip. Known advantages of scarfing include noise reduction, improved foreign object damage resistance, and improved lower lip high angle of attack performance. Known disadvantages of the scarf inlet include reduced pressure recovery and increased pressure distortion at certain low speed conditions, increased drag at cruise, and decreased upper lip flow separation margin at low angle of attack as scarf angle increases.